Thermionic tube



Jan. 7, 1941. .1. T. McNANEY 2,227,599

THERMIONIC TUBE Filed Oct. 14, 1958 fee J Jse ofi Thomas M0 lvaney.

Patented Jan. 7, 1941 UNITED STATES PATENT OFFICE 1 Claim.

My invention relates to ammeters and the like of the type utilizing thermionic tubes and it has particular relation to a novel thermionic tube capable of being utilized for potential or current measuring purposes and to a system wherein the improved tube is best employed.

It is an object of my invention to provide a potential or current measuring and indicating system of great accuracy and sensitivity, especially adaptable to the measurement of currents at radio frequencies.

Another object of my invention is to provide a novel thermionic tube, capable of functioning as a current-amplifier.

Another object of my invention is to provide a potential or current measuring system that will give an indication of either direct or alternating current.

Generally speaking, my invention resides in the provision of a novel thermionic tube having an indirectly heated filamentary cathode, to which may be supplied a current or a potential to be measured, and in the provision of a circuit such that the space current in the tube may be utilized to cause the actuation of an indicating meter or the like.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claim. The invention in its entirety,. however, both as to the organization and method of operation thereof, together with additional objects and advantages, will best be understood from the following description of a specific embodiment when read in connection with the accompanying drawing wherein:

Fig. 1 is a diagrammatic view of my improved ammeter circuit exemplifying the disposition therein of my novel thermionic tube;

Fig. 2 is a conventionalized perspective view of my novel thermionic tube, and

Fig. 3 is an exploded view of the electrodes shown in Fig. 2.

In all figures of the drawing, corresponding elements are similarly designated.

Referring now to Fig. 1 of the drawing, a preferred embodiment of my complete ammeter system includes a Wheatstone bridge in three arms of which are respectively disposed adjustable impedance devices I, 3 and 5 such as variable resistors. As is customary, a meter 1 is connected across the apices of the bridge and the usual potential source 9 is provided. The space current path in a thermionic tube ll, of novel construction, is included in one arm of the bridge by connecting a terminal of an emissive filamentary cathode l3 in the said tube to the negative pole of the potential source 9 and the anode l5 of the tube to the lower terminal of the meter 1. The emissive cathode is indirectly heated by another filament or heater l1, adapted to be ener- '5 gized from any suitable source such as a battery l9.

Before explaining the operation of my system, attention is called to Figs. 2 and 3 of the drawv ing that show my improved tube in detail. The 10 tube is preferably of the high vacuum type depending upon pure electronic emission. The cathode I3 is supported from rods 2| and 23, or the like, that are anchored into the usual press 25. The configuration of the emissive '15 cathode is immaterial and it has been shown in the drawing as a ribbon. The supports of the emissive cathode are provided, respectively, with leads 21 and 29 and which extend through the press and across which the potential to be measured may be applied as shown in Fig. 1.

The resistance of the cathode is determined by the range desired to be covered. It is preferable to provide a cathode having a resistance not higher than necessary to give the greatest change in electron emission for a definite change in temperature. The material of the cathode or the constitution of its coating, if of the coated type, as well as the anode potential, all have a bearing on the main cathode resistance chosen to provide definite predetermined sensitivity. Since these factors are well within the knowledge of those skilled in the art, they have not been referred to in more detail.

Immediately below the emissive cathode l3 and in contact therewith, I provide an insulating spacer 3|. The spacer, preferably, is constituted by a very thin sheet of mica supported from the press within the tube or carried by the tube walls. In order that the cathode may be brought to a temperature at which emission begins to show the greatest change with the slightest change in current, the heater I1 is disposed immediately below it, in contact with the under surface of the mica spacer. I have exemplified the heater as being constituted by a coil of resistance wire wound around an insulating support 33 but it is to be understood that a globar, a straight filament or any other suitable heating element may be employed if desirable.

In order that all of the electronic emission from the active cathode may be utilized, I dispose the semi-cylindrical anode l5 immediately above it, the said cathode extending along the axis thereof. Also, it is to be understood that 55 the exact shape and disposition of the anode is not material and that the construction exemplified by Figs. 2 and 3 of the drawing may be departed from radically without doing violence to the spirit of my invention. The anode may be supported from the press by a rod 35 or the like extending through an opening in the mica spacer or it may be supported in proximity to the emissive cathode in any other suitable manner apparent to one skilled in the art.

An anode lead 31 is provided which, in the embodiment illustrated, is electrically connected to the anode-support 35.

Preferably, the tube dimensions, etc., are so chosen that, after the cathode begins to emit electrons, the space current flow in the tube is directly proportional to the increase in cathode temperature.

In the operation of my improved system, the heater is first energized to raise the cathode to the point of incipient emission and the Wheatstone bridge is so balanced by adjustment of the variable impedance devices included therein that the meter registers zero. The next step is to apply the potential to be measured across the terminals of the emissive cathode or to cause a current to pass through it. The heating effect of the current flowing in the emissive cathode is independent of the direction of flow. The temperature of the said cathode, therefore, is a function of the effective current which, in turn, is a function of the effective voltage to be measured. Any increase in the temperature of the emissive cathode by the flow therein of current increases the electron emission therefrom and effectively lowers the resistance of the space current path, thus disturbing the balance of the bridge and causing the meter to give an indication.

My experiments reveal that with a 200 microampere meter in the plate circuit of my tube, it is possible to obtain a full scale deflection with less than 200 micro-amperes flowing in the cathode after it has been pre-heated to its critical point of emission. The cathode resistance can have a lower resistance than the usual thermo-couple, depending upon the plate impedance of the tube and the plate potential. Using a 1000 ohm per volt meter, this tube will provide an instrument with much more sensitivity than the most sensitive thermo-couple.

My improved system is principally advantageous in that it enables much more accurate measurement of currents or potentials at radio frequencies than when hot wire meters or thermo-couples are utilized. Also, because my novel tube is a current amplifier, it lends itself to many other uses such as power measurement, volume expanding or limiting in amplifier circuits, sound recording, etc.

I am aware that additional modifications of my invention, such as the addition of one or more grids to the tube, will be apparent to those skilled in the art to which it pertains. The invention therefore is not to be limited except insofar as is necessitated by the prior art and by the spirit of the appended claim.

I claim as my invention:

A thermionic tube comprising an anode, a cathode adjacent thereto, each end of said cathode having an individual lead-in conductor connected thereto, the conductors extending to the exterior of the tube, a resistance element disposed in intimate heat-transfer relation to the cathode, each end of said resistance element having an individual lead-in conductor extending to the exterior of the tube and insulating material disposed between the cathode and the resistance element, whereby the cathode may be raised to electron-emissive temperature by means of heat from the resistance element and potentials may be applied across the cathode independently of heating potential anplied across the resistance element.

JOSEPH THOMAS McNANEY. 

